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Chaos theory is sometimes described with an exaggerated story about the flapping of a butterfly’s wings affecting the formation of a hurricane thousands of miles away. Some “butterflies” flap harder than others, of course—a volcanic eruption can be one hell of a butterfly. According to a new study, the 1991 eruption of Mt. Pinatubo, which made a dent in the average global temperature for a couple of years, may also have a lot to do with the slower surface warming more than a decade after its eruption.

Research has made it clear that a string of La Niñas—where cold water rises to the surface in the eastern tropical Pacific—has pulled down average global temperatures in recent years. The oscillation between La Niña and El Niño conditions is a major factor in the year-to-year variability of average global surface temperatures.

So why has the coin flip come up “La Niña” so frequently lately? It appears that stronger trade winds over the Pacific—which blow westward, pushing warm surface water in the tropical Pacific as they go and bringing up the cold waters of the La Niña—are responsible. And so, you should be asking, why have the trades strengthened?

A group of researchers led by the University of New South Wales’ Shayne McGregor and the University of Hawaii’s Axel Timmermann investigated this question, as well as the cause of the accelerated sea level rise (relative to the global average) in the western tropical Pacific. They did so using a series of climate model experiments.

The first step was to see if the model could replicate major features of the observed atmospheric patterns when fed the sea surface temperature trends around the globe—which it did. The strengthening of the Pacific trade winds has come along with an increasing difference in air pressure between the Atlantic and Pacific Oceans—a sort of “seesaw” transfer. The model produced that change, as well as stronger winds and the western Pacific sea level rise.

The bulk of the experiments involved manipulating sea surface temperatures in the Indian, Atlantic, and Pacific Oceans separately to isolate the impact of each on the behavior of the atmosphere. One ocean basin was fed the observed trend between 1992 and 2011, while another was held constant at its long-term average, and the third was allowed to respond freely.

Those experiments revealed that the observed warming of the Atlantic could produce most of these interesting changes in the model. When the Atlantic warms, the air pressure seesaw tilts, which ramps up the trade winds that blow in La Niña conditions in the Pacific. The model even reproduced the drought conditions we've seen in California and the southwestern US as a result of this seesaw.

Additionally, there seems to be a positive feedback that amplifies this, as the cooler water in the eastern Pacific helps keep the North Atlantic a little warmer.

This fits in with other research showing that the climaticcycles of the Pacific and Atlantic are interrelated. Comparing records of weather through the 20th century, the researchers found that the strength of the trade winds was tightly correlated with the difference in temperature between the Atlantic and Pacific Oceans.

But recent events appear exceptional. The largest temperature difference over the last hundred years is the recent one, and the Pacific trade winds are currently the strongest on record. The Atlantic isn’t currently warming any faster than expected, Timmermann told Ars, but something unusual did happen there in the early '90s.

The eruption of Mt. Pinatubo occurred during a relatively strong El Niño in the Pacific. The Atlantic cooled off while the Pacific remained warmer. That resulted in very weak Pacific trade winds for a time. As temperatures in the Atlantic rebounded and resumed their warming trend, the Atlantic-Pacific seesaw began to swing strongly in the other direction—hitting an extreme around 2011. Without the eruption of Mt. Pinatubo, the researchers believe, that extreme swing would likely not have happened.

New York Times blogger Andrew Revkin gathered some criticism of the study from several researchers who took an initial look at it. However, in an e-mail to Ars, Timmermann seemed not to find any of them too troublesome. MIT climate scientist Kerry Emanuel’s concern about the validity of specifying sea surface temperatures in model experiments like these was probably the most relevant of those criticisms, but Timmermann said that Emanuel’s point mainly relates to middle and high latitude regions, whereas this study focuses on the tropics.

To be clear, this line of research into the fascinating interactions that drive the year-to-year variability of the Earth’s climate doesn’t affect the big picture—the climate system is warming as rising greenhouse gas concentrations trap more and more energy. Even if further research doesn't back up this study, the work still highlights the complex workings of the climate system. In order to understand the story in the Pacific, we may have to look to the Atlantic. And to understand the story in the Atlantic, we have to look to the eruption of a volcano in the Philippines more than 20 years ago.